1. Field of the Invention
This invention relates to elastomeric copolyesterethers based on poly(butylene terephthalate) modified with polypropyleneether glycol and an aliphatic or cycloaliphatic acid which unexpectedly have improved clarity when molded into articles.
2. Description of the Prior Art
Copolyesterethers are well known in the art. It is also well known that increasing the molecular weight of the polyether glycol increases the crystallization rate of the polymer. However, use of high molecular weight polyethers results in inhomogeneity of the polymerization mass during melt condensation. This incompatibility of esterether segments produces phase separation resulting in polymer compositions having inferior physical properties. It is unexpected that modification of the copolyesterether with an aliphatic dicarboxylic acid allows the use of high molecular weight polyoxypropylene glycol and prevents phase separation during melt condensation.
U.S. Patents of interest which describe polyesterethers include U.S. Pat. Nos. 3,406,045; 3,576,773; 3,580,874; 3,023,192; 3,775,373; 3,663,653; 3,775,375; 3,261,812; 3,013,914; 3,763,109; 3,651,014; 4,003,882; 4,003,883; and British 1,421,610.
Of particular interest is U.S. Pat. No. 3,763,109 which broadly discloses copolyesterethers, including those prepared from dimethyl terephthalate, 1,4-butanediol and poly(propylene oxide) glycol having a molecular above 400. A second acid, such as an aliphatic or cycloaliphatic acid, may be used. This patent, however, does not suggest the importance of using the specific polyesterether according to this invention. Furthermore, at Column 4, Lines 50-55, it is suggested that when this particular polyesterether is used, the poly(propylene oxide) glycol preferably has a molecular weight of about 600-1600. It has now very surprisingly been found that in this particular polyesterether, if poly(propylene oxide)glycol (sometimes referred to herein as polyoxypropylene glycol or polypropyleneether glycol) having a molecular weight of about 1700-2400 (preferably about 1800-2200) is used, unexpected improved clarity in articles produced therefrom is realized. Thus, higher molecular weight polypropylene ether glycol can be used to result in faster crystallization rates of the copolyesterether.